X-ray fluorescence spectroscopy permits measurement of the atomic composition of materials by observing the radiation emitted by a material when it is excited with a source of high energy photons such as x-rays or gamma rays. X-rays result when an electron is knocked out of its orbit around the nucleus of an atom by a photon from the source. When this occurs, an electron from an outer shell of the atom will fall into the shell of the missing electron. The excess energy in this interaction is expended as an x-ray photon. Since each element has a different and identifiable x-ray signature, the elemental composition of a sample can be identified.
Typically, x-ray fluorescence spectrometers include a source of radiation and a detector. The detector emits electrical impulses that are proportional to the energy of the photons being emitted by the sample. The impulses are amplified and pulses are counted from discrete portions of the sample's spectrum where x-rays emitted by the element under investigation can be found. The data is treated to isolate the x-rays being measured from other nuclear events and electronic noise with the aid of a computer.
Low-level lead poisoning is widespread among American children, afflicting as many as 3,000,000 children under age six. Lead poisoning causes intelligence quotient deficiencies, learning disabilities, hyperactivity and behavioral problems. Children can become poisoned through oral contact with the paint or through ingestion of dust containing lead paint. Upon detection of lead-based paint, the abatement process requires evacuation of the unit and removal or encapsulation of lead paint. A proper cleanup after abatement is essential since the residual dust is highly toxic. Post-abatement inspection is required prior to re-occupancy. Typically, a damp paper wipe is used to collect dust samples from the abatement site. The samples are then sent to accredited laboratories to measure their lead content.
The detection of lead-in-paint by x-ray fluorescence spectroscopy (XRF) has become the preferred method in the abatement industry because it is accurate and non-destructive. To measure lead in paint films the technician uses a portable spectrometer that has a source of gamma radiation such as cobalt 57 or cadmium 109, to irradiate the surface being analyzed, and a semiconductor detector, or a scintillation crystal coupled to a photomultiplier as the radiation detector. The technician simply holds the device against the surface for a measured amount of time and a reading is obtained. Unskilled operators can use these machines effectively. Most difficulties encountered with these devices occur when the underlying wall or molding presents an unusual backscatter spectrum, or the overlying non-lead paint layer thickness becomes great enough to cause attenuation of the underlying lead layers.
Conventional x-ray fluorescence spectrometers can produce quantitative results if either of two conditions are met: the sample is inherently uniform in analyte concentration, or the sample can be prepared such that its volume falls within the linear area of the particular spectrometers field of view. Randomly distributed analyte such as that found in dust wipe or air filter samples cannot usually be quantified without careful sample preparation. X-ray fluorescence spectrometers are inherently non-destructive, and can be made portable, repeatable, quantitative and capable of producing a hard copy result that can be validated.
It is an object of this invention to make possible the construction of a portable automatic x-ray fluorescence spectroscopy system capable of analyzing non-uniform samples of metallic elements, particularly lead in the microgram range, that are contained in or on the medium used to collect the metallic elements.